Inkjet printing technology is used in many commercial products such as computer printers, graphics plotters, copiers, and facsimile machines. One type of inkjet printing, known as “drop on demand,” employs one or more inkjet pens that eject drops of ink onto a print medium, such as a sheet of paper, to produce dots on the print medium. Printing fluids other than ink, such as preconditioners and fixers, can also be utilized. The pen or pens are typically mounted to a movable carriage that scans or traverses back-and-forth across the print medium. The print medium is advanced between scans in a direction perpendicular to the scanning direction. As the pens are moved repeatedly across the print medium, they are activated under command of a controller to eject drops of printing fluid at appropriate times. The ejection of the drops is controlled so as to form a desired image on the print medium.
An inkjet pen generally includes at least one fluid ejection device, commonly referred to as a printhead, from which the drops of printing fluid are ejected. One common printhead architecture includes a substrate having at least one fluid feed hole and a plurality of drop generators arranged around the feed hole. Each drop generator includes a firing chamber in fluid communication with the fluid feed hole and a nozzle in fluid communication with the firing chamber. A fluid ejector, such as a resistor or piezoelectric actuator, is disposed in each firing chamber. Activating the fluid ejector causes a drop of printing fluid to be ejected through the corresponding nozzle. Printing fluid is delivered to the firing chamber from the fluid feed hole to refill the chamber after each ejection. Generally, only one subset of drop generators is fired at a time to reduce peak current draw. A subset of nozzles that fires simultaneously is referred to as an “address,” and a set of adjacent nozzles containing one instance of each address is called a “primitive.”
To provide high image quality, each nozzle of the printhead should be able to accurately and repeatedly deposit the desired amount of printing fluid in the proper pixel location on the print medium. However, printhead aberrations can cause misplaced drops that vary from the desired location on the print medium, resulting in what is known as dot placement error. Such dot placement error can have a component in the direction that the carriage is scanned, which component is known as scan axis directionality (“SAD”) error. Dot placement error can also have a component in the direction that the print medium is scanned, which component is known as paper axis directionality (“PAD”) error.
Printheads are typically constructed so that the nozzles are arranged in two or more columns, each lying perpendicular to the scan axis. In some designs, the nozzles of each column are located at the same axial location relative to the scan axis (i.e., in a straight line perpendicular to the scan axis). Such a configuration is often referred to as an “inline” architecture. With inline designs, the time that elapses between firing can result in SAD error. Other printhead designs strive to reduce SAD error by employing staggered nozzle columns in which various nozzles in a column are located at slightly different locations relative to the scan axis. A staggered nozzle layout is often, but not always, accomplished by providing the drop generators with different shelf lengths. As used herein, the term “shelf length” refers to the distance, for a given drop generator, from the center of the nozzle to the edge of the fluid feed hole adjacent to that drop generator. Staggered printhead designs reduce SAD error by matching the distances between nozzles to the distances traveled by the carriage in the time between firings.
However, material deformations can occur during the fabrication of printheads with staggered designs that create systematic concentricity variations from nozzle to nozzle. These concentricity variations can cause PAD error, which is generally considered to be more problematic than SAD error because it is difficult to compensate for and leads to banding defects.